This invention relates to a clamp arrangement by which the black level of a television signal can be clamped to a reference voltage.
Prior art black-level clamps for television signals often use a series capacitor together with a gate such as a bipolar transistor, which gate is coupled to the output plate of the capacitor and to a reference voltage or ground. The gate is operated during the horizontal retrace interval to clamp the output plate of the capacitor to a reference voltage derived from the signal during an interval within the horizontal retrace pulse. This allows the capacitor to assume a charge which is retained when the gate is OPEN to produce an offset voltage across the capacitor which clamps the signal level on the output side of the clamp. In television cameras and receivers, the horizontal drive pulse encompasses a time interval during which the electron beam of the camera tube (generally a vidicon) or kinescope is retraced. The retrace is accomplished by a high voltage pulse applied to the deflection yoke to rapidly reverse the current therein. The deflection currents and associated energies are large, and coupling of unwanted signals from the deflection circuits to the signal circuits can occur. Black-level clamps which use a gate operated during the horizontal drive pulses to generate a control signal for an offset voltage generator may have the clamping level perturbed by noise signals which appear not only in the signal paths but on the ground or reference voltage point.